A rotary throttle valve carburetor comprises a carburetor main body having a fuel and air mixing passage formed therein, a cylindrical valve chamber formed in the main body perpendicular to the fuel and air mixing passage and, a rotary throttle valve received in the valve chamber for rotary and axial movement. The throttle valve includes a shaft with a through hole or passage that is increasingly aligned with the fuel and air mixing passage as the throttle valve is moved toward its open position. In such a carburetor, because the negative pressure in the fuel and air mixing passage is relatively high at idle and low speed positions of the throttle valve, fuel supply at idle and low speed engine operation tends to be excessive and difficult to control.